More Fun with Castings – NOT

[JasonBModerator @jasonb]

While putting the finishing touches to the latest engine and waiting for paint etc to dry I possibly foolishly in hindsight dug out a casting set to pass the time.

First I chucked up the cylinder liner approx 100mm long x 50mm diameter and proceeded to destroy 3 inserts. Both ends were so hard the carbide was just getting worn in seconds. Put it away thinking maybe I would heat treat it and have another go.

Next was the piston casting, decided to skim the skirt so I could hold it firmly to then machine the chucking spigot. Another insert bit the dust with the same problem of chill. Now starting to think about buying a foot of 50mm cast iron bar as heating may not work and even if it did the resulting iron was not going to be the nicest to machine for a parallel cylinder and close fitting piston.

Swapped over to the 4-jaw and thought I would have a go at the cylinder head. About 2mm of draft angle machined away reasonably well so I took a measurement and deduced that 4.5mm thickness still needed to come off the 58mm diameter flange. Started to hear an interrupted cut but pressed on even though I could see something getting bigger on the revolving machined surface. When I turned the lathe off this is what I found. The large hole goes almost to the ctr and is right where one of the three head bolts goes so little chance of getting a good head seal. As this is a hot tube engine with a lamp burning right under the head JBWeld is not going to be an option. I could CNC it from a 50mm long slice of 70mm cast iron bar but have started to loose interest with still other yet untouched castings to come.

Also if I did heat treat the other two castings what are my chances of finding holes in them if I do get under the skin?

Castings, don’t you just love them☹☹☹

[Speedy Builder5Participant @speedybuilder5]

Not that I wish you any ill feelings, but it is good to see that such an accomplished engineer as yourself experiences these difficulties now and again.  I hope the chunk of cast iron bar you purchase has a better integrity.

Bob

[noel shelleyParticipant @noelshelley55608]

I feel for you Jason ! Never mind the quality – feel the width. Noel.

[duncan webster 1Participant @duncanwebster1]

When I had some wheel castings with really hard spots the only thing that worked was brazed carbide with a big nose radius. Tips just crumbled quick time. I should have sent the castings back, but impatience ruled

[cogdobblerParticipant @cogdobbler]

It’s not just castings. I have had similar hard spots in 2″ bright mild steel bar recently.  Possibly they are slag inclusions but they are impossible to machine with carbide inserts. Steel merchant claimed ignorance but did admit to sourcing stock from that manufacturing superpower in the Far East “to keep costs down”.

[JasonBModerator @jasonb]

I’ve had that a couple of times too. I wonder if it is old carbide inserts getting thrown into the swarf and waste that then gets melted.

With a final wall thickness of 2mm the cylinder liner would likely distort under the force needed to drive even brazed carbide through the hard areas. Not so bad on something like a hefty flywheel rim but unlikely to end happily on the liner.

[Nigel Graham 2Participant @nigelgraham2]

That attack of iron-worm looks like I found in a piece of cast-iron I tried – but to be fair my raw material was an architectural item, not a dedicated engineering casting, so homogeneity would not have mattered for its original use.

Can hot-rolled mild-steel have a hard skin too, though?

Trying to drill a large hole through a short piece of standard, heavy structural channel section, went well until approaching the underside the steel appeared far harder. Possibly the drill had blunted but it did not look like that. It was if I’d come up behind a hard surface.

 

 

[David George 1Participant @davidgeorge1]

Whist still at work a few years ago, I was involved in making a mould tool to make a part for a then new Jaguar front radiator grill.  The piece of P20 pre toughened steel weighed perhaps half a Tonne started off ok but after spending over two weeks on CNC and gun drilling etc we hit a hard spot and as the cutter which was cutting a lozenge shape into the roughed out area would not cut and disintegrated under cutting load.  The metal supplier was most apologetic and it came to light that the steel piece was from the far east and not the local Sheffield. The steel was scraped and replaced but we were always under pressure for being behind schedule and as the customer, not Jaguar but the moulding company, withheld payment for the total  payment for the in total for four tools in the suite, our company ran out of cash and was put into receivership and closed down with 40 people redundant including me.

David

[Graham MeekParticipant @grahammeek88282]

Apart from the time taken to set up a casting, the chances of defects is always present. This has happened to me more than once and is one of the reasons I machine from the solid, or fabricate. Not to mention the variability of the material.

Regards

Gray

 

[noel shelleyParticipant @noelshelley55608]

One foundry I know of went to a lot of expense and trouble before they went into producing small iron castings and the spec of the raw metal was vital to success – it’s no good just melting down sash window weights ! For good castings the entire process from the mold, to the metal, to the pouring has to be right.  Noel.

[JasonBModerator @jasonb]

I did give the hard piston and cylinder liners a cook and blast with the Propane but It did not make much difference, the smaller end of the eccentric rod did soften up nicely.

I don’t have any drawings for this engine, just the castings but felt the piston was too long so managed to cure that by cutting off the hard area which is easily seen as the bright shiny metal

Jury is still out on the cylinder head, I’ve filled it with JB Weld and will do a bit more machining but as it is quite close to the blowlamp that heats the hot tube I’m a bit worried about heat. The cylinder liner I will do from 50mm cast iron bar though if anyone knows of a source of short lengths of honed hydraulic tube 44-45mm bore would also conside that as a liner.

If you are wondering it is a 1/3rd scale model of this Robinson HA, paraffin fueled

 

[noel shelleyParticipant @noelshelley55608]

The alchemists of old tried to turn lead into gold and whilst the form the carbon takes can be adjusted by heat and cooling, the heat has to be over an extended period of soak. 1000 – 1100 C.  I doubt that the barbecue was hot enough or for long enough. I can let you have 12″ of 2″ piston rod to bore out. Noel

[Clive FosterParticipant @clivefoster55965]

Jasons experiences with hard, chilled, castings illustrate the financial risks of going all in on inserts. Although the cost per edge of inserts over the putative lifetime if used appropriately is low the expense of wiping off a few low time or new edges on excessively obdurate castings soon adds up.

I agree with Duncan that an old fashioned brazed carbide tool with a decent radius tip is the way to go when faced with chilled castings. Fairly crude sharpening with flat top works well enough to chew the hard bits away. Given the inherent difficulties of making small iron castings alluded to by Noel it’s arguably wise to test everything with a file or something like the brazed carbide before moving to proper tooling.

The world is changing as we transition to insert first tooling and remote supply of castings from suppliers with less oversight so the precautions and methods we use need to change too. What was appropriate when antediluvian penguins like me were still young and hairy may no longer be the case.

These days a wrong approach tends to be financially expensive whereas for young Clive it was temporally costly. At least I’m just young enough to have pretty much avoided the carbon steel to HSS transition. Continual regrinding of an HSS tool to deal with chilled sections was, ahem, annoying. When machining allowances are sufficient an angle grinder was handy!

Clive

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